An HVAC system is the primary system for providing steady-state thermal comfort and acceptable indoor air quality in residential homes and commercial structures. HVAC systems are well known in the prior art, and function to selectively circulate conditioned air throughout a home or structure according to feedback from a thermostat. When engaged in a cooling mode, a typical HVAC unit passes air over a cooling coil and discharges conditioned air throughout the home via ductwork until a desired temperature is reached. Passing air over the cooling coil also functions to dehumidify the passing air. When the temperature of the air in the structure reaches a desired temperature on the thermostat, the unit shuts down until cool air is needed again.
An emphasis on energy efficiency can be found in every aspect of the home environment. By adding energy-efficient windows and insulation to a home, the heating and cooling loads decrease greatly. Numerous attempts have been made in the prior art to improve the energy efficiency of HVAC systems. Over the years, HVAC units have become more efficient (by government mandate). In order to manufacture these units with more efficiency, their best cost to benefit ratio was to increase the “Sensible Output” (which is the cooling BTU output side of air conditioning). This results in a decrease of “Latent Output,” which is the moisture removal BTU side of air conditioning. By increasing the coil size, an HVAC system can achieve more Sensible BTU output with less power consumed for the same given amount of airflow across the indoor coil at 400 CFM/Ton. This increases the SEER rating of the unit. However, increased coil size reduces the temperature of the coil, due to extracting more BTU's from the larger coil surface, resulting in reduction of condensation and moisture removal. Therefore, the amount of airflow, or cubic feet per minute (CFM), moving across the indoor coil is critical for dehumidification. Unfortunately, many manufactures are now recommending airflow be dropped below the 400 CFM/Ton standard to try to remove more moisture off the coil or lower the indoor temperature to over-cool the space. Both of these actions create mold control issues.
HVAC systems employing oversized equipment exhibit the unintended consequence of failing to have a long enough run time in cooling mode to extract sufficient humidity out of air in the structure, i.e. increased sensible output is achieved at the expense of latent output. As a result, insufficient airflow is delivered to the conditioned space. The current trend in prior art solutions favors energy savings at the expense of increased humidity. To amplify this problem, when super-cooled air is discharged into the humid air in the surrounding duct work, condensation occurs in the duct work. This scenario provides optimal conditions for mold growth in the duct system. In addition to mold growth, discharging humid air into the conditioned space can cause various structural issues, such as peeling paint, soggy drywall, frame and trim rot from condensation on windows, and mold or mildew growth in carpets.
Another limitation with prior art HVAC systems is that the operation of the system is based on static temperature controls. When the temperature of the home is above a desired value, the thermostat triggers a cooling call to the HVAC system to discharge cool air throughout the home. If the temperature falls below the desired value, the cooling call is cancelled, in turn shutting off the airflow. This temperature based feedback response fails to take into account the other conditions in the home a HVAC system should manage, such as humidity level of the indoor air, air exchanges per hour and air quality, and system energy consumption.
The prior art includes many examples of HVAC systems that have tried to solve some of the previously described problems. For example, U.S. Pat. No. 6,604,688 utilizes a bypass system in low load conditions that shunts return air around the cooling coil and injects it back into the system post cooling coil. The temperature of the cooling coil is increased which decreases its ability to removed latent heat and dehumidify the incoming air. To compensate, the bypass damper opens to allow all the return air to bypass the cooling coil. This allows drier, warm return air to mix with the incoming air that passed over the coil and maintain the supply air at a warmer temperature during low load conditions. However, increasing the temperature of the cooling coil decreases its ability to draw moisture from the incoming air and does not provide a substantial dehumidifying effect. In addition, this system utilizes valves to substantially close off airflow in the ducts during low load conditions, thereby maintaining a constant temperature yet failing to provide sufficient air exchange to such space.
What is needed, therefore, is a HVAC system capable of cooling and dehumidifying the air in a home, while maintaining sufficient airflow and energy-efficiency. Through applied effort, ingenuity, and innovation, Applicant has identified a number of deficiencies and problems with HVAC systems. Applicant has developed a solution that is embodied by the present invention, which is described in detail below.